Art of coating metals



Patented Mar. 26, i940 William J. Weder, Philadelphia, Pa., assignor to General Electric Company, a corporation of New York No Drawing.

1 Claim.

This invention relates broadly to the art of coating metals, and more specifically to the art of applying to; metallic elements and alloys thereof that can be electroplated with mercury 5 an electrolytically deposited coating of a metal that will amalgamate with mercury.

This application is a continuation-in-part of my'copending application Serial Number 139,207, filed April 27, 1937, which application, in turn,

is'a continuation-inpart of my copending ap- .plication Serial Number 90,882, filed July 16,

1936, both of which applications are assigned to the same assignee as the present application, now being Patents 2,094,483 and 2,094,482,

15 respectively.

In the art of coating metals that can be amalgamated with mercury for example, copper, it has been known heretofore to coat the base metal with mercury either by rubbing it with metallic 0 mercury or by chemically precipitating mercury froma mercury compound. Such methods have been described in the literature as a quicking" treatment, but hereinafter will be referred to It has also been known' as a mercury dip." g5 that'other metals that can be electrolytically deposited and which will form an amalgam with mercury may be superposed on a metal previously coated with mercury in the manner just described. Metallic coatings produced in this manner. have not been wholly satisfactory due to.

the difliculty experienced in obtaining a uniformly clean surface of the base metal prior to applying mercury thereto. By mercury dip methods the mercury coating was easily contaminated, for example with oxides of mercury,

35 and such contamination materially lessened adhesion between the mercury layer and the layer or coating of metal subsequently electrolytically deposited upon the mercury surface. Further, in practical operations the base metal, particularly when using such a readily oxidlzable metal as copper, tends to have a film of impurities, for example oxides, thereon. This film, even when present in a thickness undetected by the naked eye, hinders the formation of a complete bond ings so produced result in considerable spoilage. For example, when silver has been electrolytically deposited on copper which previously. hasbeen 5o coated Withmercury in the manner before described, spoilage due to ineffective coatings often has been as much as 15%.

In an effort to obviate the above-described difficulties it has heretofore been common pracbetween the mercury and the base metal.- Coat tice to acid dip the work, followed by scrubbing Application September 22, 1937. Serial No. 165,186

with pumice stone and water, then mercury dip, and finally to scrub the work again with a soft fiber brush in order to spread the mercury .over

, the entire surface. The process was tedious,

the results were non-uniform due in part to the 5 personal element involved, and labor costs were relatively high. a

It is an object of this invention to provide an improved method for coating metals and alloys thereof that can be amalgamated with mercury with a. continuous tightly adhering coating of another metal that can be electrolytically deposited upon a mercury surface and will amalgamate therewith.

It is a further object of thisinvention to provide 15 a method of the kind described in the preceding paragraphwhich method is economical, eliminates a large part of the personal equation in securing uniform results, and substantially decreases spoilage of the work.

In carrying this invention into' eifect I may proceed for example, as follows:

The metal article'to be coated, for example, a portion of a copper element of an electrical device such as a copper electrical switch part at the point at which electrical contact is to be made; may be cleaned'by dipping in a suitable acid solution such,- for example, as a mixture of sulfuric and-nitric acids or, under certain conditions, the copper part advantageously may be cleaned by first dipping it in a suitable alkaline solution, for instance a solution of causticsoda. After this it is water rinsed and dipped in an acid solution of the kind just mentioned. In either case, 'a-final water rinse is given the article prior to further treatment.

The cleaned surfaces of copper-or equivalent metal are then electroplated with mercury, using.

a suitable mercury-plating solution. For purpose of illustration I mention below a plating solution, 40 specifically a mercury cyanide bath, which I have used successfully in mercury plating copper, al-' though it is to be distinctly understood that other mercury-plating solutions may be employed and that thespecified percentages may be varied as desired or as conditions may require:

Mercuric chloride ounces 1 /2 Sodium cyanide do 4 Ammonium chloride do 2 Water -4--. gallon 1 A solution made in the proportions stated is put into an electroplating tank of ferrous metal such as steel, which serves as the anode. The copper parts to be mercury plated are placed as in this solution which is at room temperature. The copper articles form the cathode. Current is applied in well-known manner using, for example, a current density of from about 10 to 15 amperes per square foot. A uniform coating of metallic mercury is plated on the copper in from about 3 to 5 seconds. It is simply necessary to secure complete coverage, as any excess mercury merely falls 01f. If desired, surfaces not to be coated with another metal, and which therefore it would not be necessary to mercury plate in carrying the present invention into effect, may be masked to prevent plating thereof.

After electroplating of the mercury on the copper, the piece is rinsed in cold water and immediately thereafter is transferred to a plating bath of a metal capable of amalgamating with mercury, for example, a silver plating bath. For

purpose of illustration Imention below a suitable silver-plating solution, specifically a silver cyanide bath, although here too it is to be distinctly understood that other silver-plating solutions may be employed and that the specified percentages may be varied as desired or as conditions may require:

Silver cyanide ounces 4 Sodium cyanide do 6 Ammonium chloride do /2 Water gallon 1 that a 3 mil coating of silver will be deposited on the work in from about 2 to 2 /2 hours. The plated article is removed from the bath, rinsed inhot water and then dried in any suitable manner, for example under an air blast.

Metallic coatings produced in accordance with this invention adhere very tenaciously to the base metal. For example, two copper articles each first electroplated with mercury and then silver plated when rubbed together under pressure showed no indication whatsoever of apeeling or separation of the silver coating from the copper; instead, the silver coating merely wore through to the copper base after prolonged rubbing together of the articles, and showed no signs of galling at any time.

The mercury plating operation, which is an essential step in practicing the present invention, provides a more uniform coverage of mercury on the work than otherwise may be obtained. In addition, the mercury apparently penetrates further within the surface of the work. It has been demonstrated under practical conditions of operation that an electrolytically applied coating or film of mercury on a metal surface better proapparently combines both with the base metal and with the mercury to form oxides thereof and results in pooradhesion of the mercury coating as a whole to the base metal.

The improved results obtained by practicing this invention are due in large part to the fact that oxides are eliminated both from the surface of the base metal and from the mercury coating thereon. This elimination of oxygen is accomplished when mercury is electroplated on the base metal, in contrast with the results of the mercury dip process, since the mercury is deposited simultaneously with the liberation of hydrogen which has a reducing effect on any oxides present. In this manner what properly may be described as a-nascent action takes place, and, as a result, an improved bond or union between surfaces of the pure metals is obtained.

As more fully pointed out in my copending applications Serial Number 90,882 and 139,207, now being Patents 2,094,482 and 2,094,483, respectively, the amalgams formedbetween, for example, copper and mercury and between sheet silver and mercury are largely responsible for the strong bonds formed between such amalgamated metals under pressure or under heat and pressure. The formation of such amalgams has a similar beneficial effect in providing tenaciously adhering coatings that are produced in accordance with.

this invention.

In practicing my invention the coating metal,

erations commonly employed heretofore both before and after a mercury dip of the base metal are eliminated. Further, there is practically no spoilage whatsoever due to imperfect coatings.

Examples of metals which may be used in carrying this invention into eifect are copper, silver, lead, tin; gold, cadmium, bismuth, antimony and alloys thereof. Such metals may serve either as the base metal or as the coating metal. A preferred embodiment of the invention consists in silver plating copper with the aid of an intermediate layer of electrolytically deposited mercury.

Of course it will be understood by those skilled in the art that an acid .dip or a combination of an alkalineand acid dip results ina brightening of the metal surface.

The terms metal and metals as used hereinbefore include both substantially pure metal of the particular genus or species stated and alloys thereof.

What I claim as new and desire to secure by Letters Patent of the United States is:

The method of making an electrical contact point on a copper element of an electrical device which comprises cleaning at least that portion of the said element where electrical contact is to be made, electrolytically depositing a coating of mercury from a mercury cyanide bath upon the cleaned portion and electrolytically de- 'positing a coating of silver from a silver cyanide bath upon the mercury coating.

WILLIAM J. 

